The present invention relates to an ultrasonic lithotrity apparatus which is capable of focusing an ultrasonic wave to break up a stone formed in a living body.
An apparatus which breaks up a renal calculus or a gallstone formed in a living body, by externally supplying shock wave energy generated by a discharge or explosion of a powder, is well known, as disclosed in U.S. Pat. No. 3,942,531.
However, as an alternative to the apparatus utilizing shock wave energy, an apparatus for breaking up a stone by means of ultrasonic energy has recently been put into practical use. This apparatus is small in size, can be manufactured at low cost, and can easily determine the presence/absence of a stone.
For example, as is disclosed in U.S. Pat. No. 4,617,931, a well known ultrasonic lithotrity apparatus is constituted by a lithotrity transducer and an image transducer. The lithotrity transducer is concave in shape, forms the focal point of an ultrasonic wave at its geometrical center, and breaks up a stone by focussing emitted ultrasonic energy thereto. The image transducer is used to obtain tomographic image data of a patient; that is, the image of a stone, as part of a tomographic image obtained by the image transducer, is positioned so as to coincide with the geometrical focal point of the lithotrity transducer. After such a coincidence is detected, a wave of intense ultrasonic energy is emitted from the lithotrity transducer and is focused onto the stone, thereby breaking up the stone.
However, when this conventional ultrasonic lithotrity apparatus is used, refraction of an ultrasonic wave is generated on part of the body surface of the patient, or even in the body itself. Therefore, the focal point within the patient, at which an ultrasonic wave emitted from the lithotrity transducer is focused, does not always coincide with the actual position of the stone, as determined by the image transducer. Consequently, the ultrasonic wave is sometimes undesirably focused onto a position other than the actual position of the stone, thereby posing a problem as regards the safety of the patient.
In addition, a stone is rarely broken by only one emission of ultrasonic energy; therefore, ultrasonic energy is normally emitted a number of times. During a series of ultrasonic energy emissions, the patient may sometimes move during repeated operations, with the result, that the focal point of the ultrasonic wave may deviate from the actual position of the stone. Thus, as in the above case, the ultrasonic wave is undesirably focused onto positions other than that of the stone, with possible adverse effects on the patient's health.